The 28-year-old actor added: "I would never have the balls to do it in a million years."

Show insiders said that the judges - Simon Cowell, Cheryl Fernandez-Versini, Mel B and Louis Walsh - were won over by Lizzy's soulful voice, and that she initially kept quiet about her celebrity connection.

"It was really important Lizzy was judged for her voice on the show and not as Rob's sister - that's why she hid the link at first," the source said.

"But she couldn't keep it under wraps for long. During her audition she was asked about him but all she said was he was very supportive. She's not the kind of person to cash in on the connection. However far she goes, she wants it to be on merit."

The new series of The X Factor, with Simon and Cheryl returning to the UK panel for the first time since 2010, begins on August 30. It will also be shown on US screens.